In the present invention, an electromagnetic wave absorbing material is defined as a material capable of absorbing electromagnetic waves, and an electromagnetic wave absorber is defined as a product (an article) composed of an electromagnetic wave absorbing material designed to absorb electromagnetic waves at a specific wavelength.
Practical research into the electromagnetic wave absorbing materials is believed to have been disclosed first in A. J. Simmons and W. H. Emerson, I.R.E., National Conv. Record, 1953. Since then, much research has been done and various electromagnetic wave absorbing materials have been reported.
A carbon-containing material is a material obtained by incorporating carbon particles into urethane foam, styrene foam, rubber or the like, and this is an electromagnetic absorbing material utilizing the electrical conductivity loss of carbon. This material can absorb electromagnetic waves in a relatively high frequency band and, therefore, can be used in an anechoic chamber or the like. However, as the carbon is in the form of electrically conducting particles, reflection from the particle surfaces cannot be avoided. In order to solve this problem, a method of forming an electromagnetic wave-capturing layer by providing a pyramid-like layer or by overlapping materials differing in the particle content density in layers to provide an electromagnetic wave incident surface having a composition close to air is employed. Accordingly, in the case of using carbon fine particles, there is a problem that even a relatively thin pyramid-type electromagnetic wave absorbing material for 1 GHz has a thickness of 200 mm or more and the service space is limited.
Ferrite is a electromagnetic wave absorbing material utilizing the magnetic loss. This is also used by incorporating it into styrene foam, rubber or the like, but this material absorbs electromagnetic waves only in a narrow band of a relatively low frequency (100 MHz to 1 GHz) and is not suitable as an electromagnetic wave absorbing material for a future high frequency band. Furthermore, the main component is iron and therefore, there is a problem that the density is from 5 to 10 g/cm2 and the weight is high as compared with other electromagnetic wave absorbing materials.
A λ/4-type electromagnetic wave absorber is obtained by attaching a resistive film to the surface of a lossless dielectric material lined with a metal sheet. The thickness of the lossless dielectric material is ¼ of the wavelength, and this material does not absorb electromagnetic waves in a wavelength other than the object wave length. Therefore, this is an electromagnetic wave absorber for a very narrow band.
As for the relationship between the absorbing bandwidth and the thickness of existing electromagnetic wave absorbing materials, the tendency in general is such that, when the adsorbing bandwidth is wide, the material is thick and when the material is thin, the absorbing bandwidth is narrow. An electromagnetic wave absorbing material having a small thickness and a wide absorbing bandwidth has not yet been developed.
In order to solve these problems, Japanese Unexamined Patent Publication (Kokai) No. 2003-133782 describes a electromagnetic wave absorber comprising a composite material of electrically conducting inorganic substance-containing inorganic fiber and an insulating matrix, in which the fiber has a composite phase consisting of a silicon carbide-based inside and an electrically conducting inorganic substance surface layer, the compositional ratio of the electrically conducting inorganic substance gradiently increases toward the surface layer of the fiber, the gradient layer of the electrically conducting inorganic substance has a thickness of 1 to 500 nm, and it can selectively absorb electromagnetic waves of 1 to 300 GHz in a wide band. It is indicated that the electromagnetic wave absorbing band and the band width can be arbitrarily controlled by selecting the fiber content or thickness of the electromagnetic wave absorber, or the thickness of or orientation in the surface electrically conducting layer of the fiber used therefor, and/or selecting the fiber content or thickness of the electromagnetic wave transmitting member laminated on the incident surface, or the resistivity or orientation of the fiber used therefor.
The electromagnetic wave absorbing material described in Japanese Unexamined Patent Publication (Kokai) No. 2003-133782 is advantageous, for example, in that the material can also function as a structural material by use as a fiber-shaped material but as only use as fibers is possible, there is a problem that the material itself is very expensive. Furthermore, the production of the electromagnetic wave absorber requires very complicated steps or a special technique and, for example, the fiber must involve the step of forming into a fabric shape at the compounding or must be homogeneously and neatly oriented, and this or other requirements give rise to elevation of the cost of the electromagnetic wave absorber. Also, the fiber-shaped material makes it difficult to fabricate a complicated shape. In addition, the electromagnetic wave absorber composed of a fiber-shaped material has a problem in the surface smoothness, mechanical isotropy, recycling of the electromagnetic wave absorbing material, and the like.
If the electromagnetic wave absorbing material can be formed as fine particles, the step of the shaping into a fiber form is not necessary and the cost of the electromagnetic wave absorbing material can be reduced. The electromagnetic wave absorbing material in a fine particle shape can be easily kneaded with a matrix material such as a resin or a ceramic and processed into a complicated shape by a general shaping technique, so that the production cost of the electromagnetic wave absorber can be reduced. Furthermore, the fine particle shape is advantageous, for example, in that a electromagnetic wave absorber assured of surface smoothness or mechanical isotropy can be obtained or recycling of the electromagnetic wave absorbing material is facilitated.
By taking account of these problems, the present inventors have made intensive studies to obtain a electromagnetic wave absorbing material in a fine particle shape and, as a result, there has been developed a electromagnetic wave absorbing material of electrically conducting-inorganic substance-containing silicon carbide-based fine particles having a structure such that the particle inner portion is composed preferably of silicon, carbon, either titanium or zirconium, and oxygen and an electrically conducting inorganic substance layer mainly comprising carbon is present on the particle surface, in which the compositional ratio of the electrically conducting inorganic substance gradiently increases from the inside to the surface of the particle, the electrically conducting inorganic substance layer has a thickness of 1 to 500 nm and the electrically conducting-inorganic substance-containing silicon carbide-based fine particles have a high sphericity. The present invention provides a electromagnetic wave absorbing material comprising silicon carbide-based fine particles and an insulating matrix, the silicon carbide-based fine particles having a high sphericity, excellent kneadability with resin or the like and excellent electromagnetic wave absorbing property in a wide band, in which the particle inner layer is preferably composed of silicon, carbon, either titanium or zirconium, and oxygen and a layer rich in an electrically conducting inorganic substance mainly comprising carbon is present on the particle surface; a production method thereof; and a electromagnetic wave absorber allowing for processing into a complicated shape.